1. Field of the Invention
The present invention relates to a continuous V-belt transmission having a belt made to run on primary and secondary pulleys each composed of two sheaves and, more particularly, to a shift control unit for a continuous V-belt transmission to be suitably mounted on an automobile, for shifting the primary and secondary pulleys.
2. Description of the Prior Art
In recent years, with demands for improving the fuel consumption rate and the drivability, there has been noted an automatic transmission for an automobile, which is equipped with the continuous V-belt transmission.
In the prior art, we have proposed continuous V-belt transmissions in Japanese Patent Laid-Open Nos. 13853/1987, 159847/1987, 159848/1987 and 154848/1990, for example. These continuous V-belt transmissions change the speeds by using mechanical actuators such as ball screw units as actuators for moving the movable sheaves of the primary and secondary pulleys in the axial directions, by equipping the two pulleys with pressure regulating cam mechanisms for applying belt clamping forces corresponding to a transmission torque, by associating the primary and secondary actuators through power transmission units such as gears, and by connecting shift control means to the power transmission units to rotate the primary and secondary actuators relative to each other.
Especially in the continuous V-belt transmission disclosed in Japanese Patent Laid-Open No. 159847/1987: the primary ball screw unit has its externally threaded portion formed in the movable sheave of the primary pulley; the externally threaded portion is connected through the carrier of the planetary gear mechanism to the externally threaded portion of the secondary ball screw unit; the primary ball screw unit has its internally threaded portion connected through the ring gear of the planetary gear mechanism to the internally threaded portion of the secondary ball screw unit; and the shift control means is connected to the sun gear of the planetary gear mechanism.
As a result, in an unshifted state, the primary ball screw unit rotates together with the pulley, and the rotations of the externally threaded portion and the internally threaded portion of the ball screw unit are transmitted through the individual elements of the planetary gear mechanisms to the externally threaded portion and the internally threaded portion of the secondary ball screw unit thereby to maintain a predetermined shifted state. By rotating the sun gear of the planetary gear mechanism on the basis of the operation of the shift control means, moreover, the primary and secondary ball screw units are rotated relatively to each other to establish a desired shift value.
As a result, in this continuous V-belt transmission, when the primary pulley and the secondary pulley are in the one-to-one relation, that is, rotating at an equal r.p.m., the secondary pulley and the ball screw unit are in the equal r.p.m. sate like the primary side so that a plurality of thrust bearings arranged for bearing the axial force of the pulley by the shaft are integrally rotating. In case the primary pulley and the secondary pulley are not in the one-to-one relation, their relative rotations are lower to reduce the loading capacity of the bearings the more than those of the construction in which the ball screw unit has one of its externally threaded portion and the internally threaded portion blocked from any rotation.
The continuous V-belt transmission described above has a smaller loading capacity of the thrust bearings than that of the construction in which the mechanical actuator such as the ball screw unit does not rotate together with the primary and secondary pulleys. At the secondary side, the thrust bearings arranged for bearing the axial force of the pulleys by the shaft have to be given a capacity more than that for allowing the relative rotating load. Thus, the continuous transmission has its transmission efficiency dropped to an extent corresponding to the spin lose of the bearings, and a large bearing space is required to obstruct the compact construction.
Moreover, since both the primary and secondary ball screw units are always in an associated state to transmit the rotation, it is impossible to interpose any nonlinear transmission means between the two ball screw units. As a result, at each torque ratio, the stroke difference specified by the belt between the primary movable sheave and the secondary movable sheave cannot be absorbed by the movements of the ball screw units. In the aforementioned continuous transmission, therefore, the pressure regulating cam mechanism takes a large stroke to establish the torque fluctuation and to deteriorate the durability and performance of the pressure regulating cam mechanism when the positive and negative torque are switched to a negative torque transmission for an engine braking.